Method for determining the time and extent of maintenance operations

ABSTRACT

In a method for defining the time and scope of maintenance operations for a system having a plurality of maintenance items, each of which should be carried out within an assigned tolerance range, a minimum maintenance interval between successive maintenance operations is predefined. The timing of a maintenance operation is predictively fixed at at least that tolerance range end point which is the first of the tolerance range end points of maintenance items relating to maintenance times to follow the time of a preceding maintenance operation while complying with the minimum maintenance interval. At least those maintenance operations whose tolerance range end points occur before the predicted subsequent maintenance time are defined as the scope of the imminent maintenance operation.

BACKGROUND AND SUMMARY OF THE INVENTION

This application claims the priority of German patent document 101 29457.3, filed 19 Jun. 2001 (PCT International Application No.:PCT/EP02/06479), the disclosure of which is expressly incorporated byreference herein.

The invention relates to a method for defining the time and scope ofmaintenance operations for a system having a plurality of maintenanceitems, each of which should be carried out within an associated flexiblemaintenance interval and tolerance range, with a predetermined minimummaintenance interval between successive maintenance operations.

Such methods are customary, for example, for the maintenance of motorvehicles. In this application, fixed maintenance intervals in the formof corresponding time intervals or mileage intervals are conventionallypredefined. In addition, there is usually a fixed predefinition of whichmaintenance items are to be carried out during a respective maintenanceoperation. With this type of predefined maintenance of motor vehicles,the respective vehicle component is maintained at fixed intervalsindependently of the severity of its actual wear, which may varyappreciably from vehicle to vehicle, for example due to differentdriving styles.

German patent document DE 31 10 774 A1 discloses a method for definingmaintenance times for motor vehicles in which a reference variable (forexample, the state of the brake linings or the state of the engine oil),is fixed as decisive. An associated maintenance value of the referencevariable is predefined and the actual value of the reference variable issensed continuously while the vehicle is actually operating, andcompared with the maintenance value. As soon as the actual value reachesthe maintenance value, an indication is given that a maintenanceoperation should be carried out.

For further operating variables which are to be maintained as a functionof wear, such as clutch, carburetor setting, spark plugs, ignition timesand battery voltage, their actual values are also sensed from time totime and compared with stored wear limiting values. Depending on thewear state, the respective operating variable is assigned to amaintenance operation which is determined by the reference variable,within a tolerance range which is formed as a function of the mileage,fuel consumption, time or a combination of these variables. Here, themaintenance time is defined within the tolerance range in the directionof the upper or lower range limit by reference to an evaluation of thereference variable and the respective operating variable. In addition,assuming that loading of the vehicle remains the same, the wear limitcan be extrapolated from a computing unit which carries out the method.A load diagram can be created for the operating variable values whichare decisive for the loading of the vehicle, from which diagram it ispossible to detect whether the vehicle is being operated mainly in thepartial load mode or full load mode.

German patent document DE 32 34 727 A1 discloses a method for definingmaintenance times for a motor vehicle in which the current wear ofcomponents that are to be maintained and the duration of operation or ofthe vehicle, the engine speed and the temperature of the cooling waterare measured and the current wear is compared with a predefinable wearlimiting value, in order to calculate the expected service life of therespective component therefrom. The shortest time period or distance inwhich a plurality of monitored components are subject to wear within apredefinable maximum tolerance range is then indicated, or the distancefor a component which is worn by more than the predefined tolerancerange before the other components is indicated.

One object of the invention is to provide a method of the type describedabove, with which the time and scope of maintenance operations can bedefined for a system having a plurality of maintenance items in acomparatively reliable, flexible and cost-effective way.

This and other objects and advantages are achieved by the methodaccording to the invention, in which, on the one hand, a minimummaintenance interval for a next maintenance operation (i.e., subsequentmaintenance operation) and, on the other hand, flexible maintenanceintervals and tolerance ranges for the maintenance items are predefined.The maintenance interval variable and the maintenance intervals andtolerance ranges for the maintenance items can be predefineddifferently. For the sake of optimization, it is possible to iterateover this variable maintenance interval.

Furthermore, at least some of the maintenance items are treated asserving to define maintenance times, and are referred to below ascontrol function maintenance items. The latter are taken into account indefining the maintenance times. Specifically, for a particularmaintenance operation which is to follow a preceding maintenanceoperation by at least a minimum maintenance interval, the time ispredictively fixed at least at the earliest tolerance range end point(among the control function maintenance items) which complies with theminimum maintenance interval. All maintenance items whose tolerance endpoints occur before this predicted, aimed-at subsequent maintenance timeare included in the extent of the preceding maintenance operation.

This procedure permits components which are subject to wear to bemaintained sufficiently promptly, and thus reliably, in a very flexibleway, by means of fixed or variable predefinition of the minimummaintenance interval and of the maintenance intervals and toleranceranges which can be selected individually for each maintenance item. Themaintenance intervals and tolerance ranges can be selected in a variablefashion as a function of the current conditions (in particular thecurrent measured or predicted wear of the system component or componentsaffected by a particular maintenance item), which permits a furtherimproved adaptation of the subsequent maintenance operations to thecurrent wear state of the various system components. In motor vehicles,different degrees of wear depending on the driving style can thus betaken into account in arriving at favorable maintenance times.

The method according to the invention thus ensures, on the one hand,that each maintenance item is carried out sufficiently frequently sothat worn system components are maintained promptly, and, on the otherhand, selecting a correspondingly long minimum maintenance intervalavoids premature performance of maintenance operations.

In one embodiment of the invention, the maintenance items arecategorized into one or more respective maintenance secondary items.When the sequence of a maintenance operation is created, the maintenancesecondary items of the maintenance items which are to be processed inthe maintenance operation are tested with respect to the possibility ofcombining them. This permits an effective maintenance sequence in whichthe maintenance secondary items are combined for processing in such away that, as far as possible, each maintenance secondary item has to becarried out only once.

In another advantageous embodiment of the invention, an optimizationalgorithm is used in which the complexity of the maintenance serves as aso-called cost function which is to be optimized. The complexity of themaintenance may be enumerated here, for example, as a specific amount ofmoney or a cost value.

It is possible to use maintenance positions (that is, the timing ofmaintenance items) with a control function which are extracted from theoptimization algorithm in order to determine the maintenance time, whileothers are then merely added to the maintenance packages. It is possibleto take into account fixed regulatory deadlines such as for TÜV[Germanstandards testing authority], ASU[German exhaust gas test] andnonscheduled visits to the workshop. The optimization process alsoincludes provisions for moving subsequent maintenance positions forwardon a test basis (i.e., maintenance positions, which according to thenormal criteria would not be due until a subsequent maintenance time),to a preceding maintenance operation. In this case, the predefinedminimum maintenance interval functions as a secondary condition of theoptimization process. This measure can be used to determine whether theperformance of one or more maintenance items will lead to a loweroverall degree of complexity and is therefore to be recommended.

In practice, the wear of components which are to be maintained may besubject to time intervals or distance intervals and be calculated fromload collectives or sensor data. On a program-internal basis,calculations are preferably carried out only with one unit (time ordistance) and the result can then be presented again in both units. Thepossibility of incorporating various models for determining wear forthis purpose ensures that a framework algorithm is provided. Thispermits, inter alia, wear models of suppliers to be incorporated andtested. For reasons of practicability, the system preferably permitsmanual correction of the optimum solution, for example in order toachieve further, less than optimum, solutions with associatedmaintenance deadlines, extent and cost of maintenance, as well assubsequent maintenance deadlines and costs. The user can thus predefinethe time period between two maintenance deadlines in a reasonable rangeand control weighting which is optimized to a greater extent in terms ofcost or time.

Other objects, advantages and novel features of the present inventionwill become apparent from the following detailed description of theinvention when considered in conjunction with the accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a block diagram of a basic implementation of an embodiment ofthe method according to the invention for optimized determination of thetime and scope of maintenance operations for a motor vehicle;

FIG. 2 is a schematic time or distance line illustration with variousmaintenance positions illustrating the definition, according to themethod, of the times for an imminent maintenance operation and asubsequent maintenance operation and of the scope of the imminentmaintenance;

FIG. 3 is a block diagram of a more precise structure of the maintenanceitems;

FIG. 4 illustrates testing for the possibility of combining maintenancesecondary items of two maintenance items which are taken by way ofexample, on the basis of the maintenance item structure in FIG. 3;

FIG. 5 is a schematic illustration of an optimization operation fordefining the extent of a respective maintenance operation withmaintenance items being brought forward on a test basis;

FIG. 6 is a schematic flow chart of an embodiment of the methodaccording to the invention;

FIG. 7 shows a graphic operator control interface of a working itemeditor of the method;

FIG. 8 shows a graphic operator control interface of a working stepeditor of the method;

FIGS. 9 to 11 show a percentage, time-related or mileage-related graphicdisplay of the state of maintenance items;

FIG. 12 shows a graphic operator control interface and display interfacefor manually predefining a minimum maintenance interval and displayingresults of the optimization operation and the formation of maintenancepackages; and

FIGS. 13 and 14 show graphic displays of the optimization result byiteration over minimum maintenance intervals.

DETAILED DESCRIPTION OF THE DRAWINGS

FIG. 1 shows an example of the implementation of an advantageous methodexample which is suitable for the optimized definition of the time andextent of maintenance operations for motor vehicles in the scope of avehicle maintenance function 7. As is apparent from FIG. 1, for thispurpose, a diagnostic algorithm 5 for the entire vehicle is able toaccess three modules, including specifically a travel data memory 1,results of a telediagnosis 2, which are supplied to the system carryingout the method, and a framework algorithm 3 which has models 4 ₁, . . ., 4 _(n) (n>1) for predictively diagnosing the wear of a multiplicity nof vehicle components such as engine oil, engine oil filter, sparkplugs, tracking rod/longitudinal rod, brake fluid, brake disks and brakelinings, which are to be maintained. The various maintenance item models4 ₁, . . . , 4 _(n) may be implemented in a conventional way (whichtherefore need not be explained further here), by corresponding sensors,relatively simple algorithms or relatively complex wear-determiningalgorithms (for example ones which operate on the basis of loadcollectives and which each apply their measured or calculated results tothe framework algorithm 3). The diagnostic method level 5 is connectedvia a customary man/machine interface (MMI) 6 to the vehicle maintenancefunction 7 in order to permit user inputs and display the obtainedresults to the user.

FIG. 2 is a time or distance line which illustrates the basic principle,according to the invention, of the definition of the time and scope ofthe successive maintenance operations. As is customary in motorvehicles, there are a multiplicity m of maintenance items W₁, . . . ,W_(m) (m>1) for maintaining the vehicle-mounted components which aresubject to wear. Each of these is assigned a maintenance interval withan individually definable tolerance range Δ (shown in FIG. 2, forexample, with respect to maintenance item W₅), within which interval therespective maintenance item should be carried out. At least some of themaintenance items W₁, . . . , W_(m) are selected as a respective controlfunction maintenance item which have a control function for thedefinition of the maintenance times, i.e. are considered as relevant.The respective tolerance range Δ may be the smallest common interval.

Starting from a current time t₀, that tolerance range end point E_(a)which occurs first among all the tolerance range end points of controlfunction maintenance items following to is then used as the time t_(a)for a next imminent maintenance operation. Furthermore, a minimummaintenance interval a_(min) between each two successive maintenanceoperations is preferably predefined in a variable fashion by the systemor by the user. The time t_(g) which occurs at expiration of thisminimum maintenance interval a_(min) following the time t_(a) for theimminent maintenance operation then forms the earliest possible time fora next maintenance operation (i.e., a subsequent maintenance operation).The latter is preferably also displaced as far as possible in the latedirection starting from this time t_(g) in order to reduce expenditureon maintenance. That tolerance range end point E_(f) which, of all thetolerance range end points of the control function maintenance items, isthe first following the earliest possible subsequent maintenance timet_(g), or corresponds to it, is then defined as the latest possiblesubsequent maintenance time t_(f). This latest possible subsequentmaintenance time t_(f) is defined as a predicted, desirable time of thesubsequent maintenance operation.

The execution of at least those maintenance items whose tolerance rangeend points occur before the scheduled subsequent maintenance time t_(f)(in the example in FIG. 2, these the maintenance items W₁ to W₅), isthen defined as the extent of the imminent maintenance operation.Maintenance items whose tolerance range end point is at or after thescheduled subsequent maintenance time t_(f) can be included in the scopeof the next subsequent maintenance operation, or of a later one. (In theexample in FIG. 2, these are the maintenance items W₆ to W₉). The latteralso applies to maintenance items whose tolerance range comprises theentire range between the imminent maintenance time t_(a) and thescheduled maintenance time t_(f), as in the case of the maintenance itemW₆ in FIG. 2.

A special treatment can be provided for specific special maintenanceitems which are determined, for example, by legal requirements such asthose of standards testing authorities and exhaust gas monitoring. Ifsuch a special maintenance item is the one which determines amaintenance time in accordance with the rules explained above, othermaintenance items are normally delayed to the corresponding time of thespecial maintenance item (i.e., at the latest to the time of thetolerance range end point of the special maintenance item if one existsfor the special maintenance item). If the maintenance item whichdetermines the normal maintenance time is not a special maintenanceitem, it is checked whether the latter is located among the maintenanceitems which have been brought forward to the next maintenance operation.In this case, the special maintenance item is provided with a separatetreatment. The regulatory tolerance range in which an examination isintended to take place is generally small as it is generally not desiredfor the user to move it forward owing to the associated expenditure.Therefore, if the tolerance range of the respective special maintenanceitem does not overlap with the maintenance time which is determined asexplained above, the method assigns, as a special treatment, its own,separate maintenance deadline to the special maintenance item, as far aspossible at the end of its tolerance range, as indicated in FIG. 2 bythe dot-dash line.

FIG. 3 illustrates the typical, presently used maintenance itemstructure. Each maintenance item W_(i) is composed of one or moremaintenance secondary items U_(i1), U_(i2), . . . , U_(ik) which eachrepresent a specific maintenance working step. For example, amaintenance item “Change brake lining” can thus be composed of the threemaintenance secondary items “Open brake”, “Change brake linings” and“Close brake”.

The maintenance secondary items U_(i1), . . . , U_(ik), and thus alsothe respective maintenance item W_(i) are assigned maintenance-relatedparameters such as the working time which is required for implementationand the working value which is associated with it (i.e., the costsincurred). A residual running time parameter indicates how long it isexpected to remain possible to wait for the execution of the maintenanceitem W_(i).

In the method according to the invention, this maintenance itemstructure is used to examine, during the definition of the extent andsequence of a respective maintenance operation, the maintenance itemswhich are involved and are to be carried out, to determine whether, andto what extent, they contain common maintenance secondary items and canthus be combined in such a way that a maintenance secondary item whichoccurs in a plurality of maintenance items needs to be carried out asfar as possible only once. FIG. 4 illustrates this measure using theexample of a first maintenance item “Change braking lining” which iscomposed of the maintenance secondary items “Open brake” (A), “Changebrake linings” (X) and “Close brake” (B), and of a second maintenanceitem “Change brake disk” which is composed of the maintenance secondaryitems “Open brake”, “Change brake disk” (Y) and “Close brake”. At thesame time, this example is used to illustrate the case in which thebrake lining with its working sequence AYB of the maintenance secondaryitems would in fact not have to be changed until during a subsequentmaintenance operation, but is brought forward on a test basis to animminent maintenance operation within an optimization operation(explained in more detail below). This maintenance operation includesthe changing of the brake disk with its sequence AXB of the maintenancesecondary items. Here, the term “Service” in FIG. 4 stands as a synonymfor the term “Maintenance operation”.

According to the invention, in testing the possibility of combining themaintenance secondary packages AXB, AYB, . . . of the maintenance itemswhich are to be carried out in the respective maintenance operation, arecombined as far as possible, combinatorially, after which multipledenominations of identical maintenance secondary items are deleted. Thatis, the working sequence AXB+AYB which is not combined can be simplifiedto form the combined sequence AXYB. The cost gain in terms ofoptimization which can be achieved by eliminating multiple executions ofmaintenance secondary items may, depending on the situation, exceed thenegative cost effect which is associated with moving maintenance itemsforward, so that moving them forward in such a way may have an overalladvantage in certain cases.

The cost function which is used for the optimization process may be, inparticular, a specific amount of money or invoice amount which is linkedto the respective maintenance working step. The negative cost effect asa result of moving forward results from the failure to use the estimatedresidual running time of the maintenance item, for example measured from100% to 0% related to the maintenance item costs which are predefinedfor this at the maintenance secondary item level, as explained withrespect to FIG. 3 above. The overall maintenance costs for a maintenanceoperation are then simply determined as the sum of the costs of all themaintenance secondary items to be carried out, and form the costfunction to be minimized in the optimization process.

FIG. 5 illustrates the optimization process which is used. The startingpoint here is first a complete list L1 of all maintenance items, a listL2 of all maintenance items which are to be carried out in a current,imminent maintenance operation, and their maintenance secondary items,and lists L3, L4 of the maintenance items which are to be carried out insubsequent maintenance operations, and their maintenance secondaryitems. The initial lists L2 to L4 for the current maintenance operationand the subsequent maintenance operations are created in accordance withthe procedure explained above with respect to FIG. 2.

FIG. 6 is a schematic flow chart of the method according to theinvention. The required initial variables are defined in an initialinitialization phase S1. This comprises initializing the maintenanceitem list.

Furthermore, the next maintenance time is defined with reference to thecontrol function maintenance item which is due at the earliest, and aninitial assignment of the maintenance items to the imminent maintenanceoperation and to subsequent maintenance operations is carried out inaccordance with a first optimization process (step S2).

During this optimization, iteration is carried out over the minimummaintenance interval a_(min) (for example in monthly steps). FIG. 5shows the state for an iteration step (i.e., for a specific minimalmaintenance interval value a_(min)). Which maintenance items occur inwhich regions is determined here, the list of the working steps isformed and the multiply-occurring steps deleted. The costs and the costsaving due to deletion, advancement of the maintenance items to therespective deadline and the losses due to failure to exploit residualoperation time, are calculated.

Then, there is an additional test to determine whether gains due to theadvancement of maintenance items to earlier deadlines bring savings.(See step S3 in FIG. 6.) Given work-intensive maintenance items, it isalso possible to achieve further gains here, preferably when there is arelatively small minimum maintenance interval a_(min).

The elements of a list are tested for “possibility of advancement” untilthere is no more gain at any maintenance item when passing through thelist. If a maintenance item can be moved forward, once the list hasended it is started again from the beginning as the items which havealready been tested could then possibly be combined satisfactorily withthis new maintenance item after the moving-forward operation.

The actual costs for a fixed maintenance interval a_(min) result fromthe sum of the working step costs (FIG. 5), plus the standing costs, thelatter being made up of a fixed cost element and a time-dependentelement. The duration of a maintenance deadline is calculated from thesum of the durations of the working steps, see FIG. 5.

The sum of the cost saving from combining maintenance items, on the onehand, and standing costs plus wasted money by advancing maintenanceitems, on the other, is the variable which determines the optimum interms of the peripheral condition of a predefined minimum intervalbetween two maintenance deadlines.

If iteration is carried out over the minimum maintenance intervala_(min) (for example in monthly steps) and if the above variable isplotted “per time unit” against the minimum maintenance intervala_(min), the minimum of this map or this cost function thus supplies theoptimum maintenance scope and, using a_(min), the optimum deadline andscope of a subsequent maintenance. The variable “per time unit” takesinto account, for example, the case in which one expensive maintenanceprocedure per year may be more favorable than two somewhat cheapermaintenance operations. The above variable which determines the optimumis thus to be taken correctly per time unit (i.e., to be divided bya_(min)).

The position of the extreme values, here the minimum values, isessential for a cost function. If the constant value of the sum of:“costs of each maintenance item divided by the respective currentmaintenance interval of this maintenance item” (that is to say the sumof “costs per maintenance item/time”) is added to the above costfunction, the position of the absolute minimum of the cost functionremains unchanged and the same optimum result is thus obtained. However,the function value of the cost function also supplies the actual costsof the maintenance for the respective result a_(min) includingcorrections for combination and losses due to moving forward and downtimes, per time unit.

A maintenance interval of a maintenance item is to be considered here asconstant only at the current time as it may be the result of loadcollectives or a sensor system and thus be flexible and may in turn thuschange, for example as a result of changes in the driving behavior. Thisis the basis of the person-specific or driving-behavior-specificmaintenance.

The curve may have not only the absolute minimum but further relativeminimums (less than optimum solutions of the problem) or further pointsof interest. This requires the possibility of manually inputting a_(min)including the representation of the extent of maintenance, maintenancecosts, working steps, etc., see FIG. 12.

FIGS. 7 to 14 illustrate implementation of the method using graphicscreen interfaces. FIG. 7 shows a maintenance item editor in which, forexample, the maintenance item “Rear disk brakes” in the menu item “Name”is called. At this menu item, the complete list of all the createdmaintenance items is stored. Maintenance items can be edited or newlycreated using the maintenance item editor. Working steps listed in theleft-hand lower window may be assigned to each maintenance item. Theright-hand lower window shows the working steps assigned to therespective maintenance item. Furthermore, each maintenance item containsan attribute “Control function”, “Additional maintenance item” or “Onlymonitoring”, as well as the possibility of inputting the maintenanceinterval and tolerance range of the maintenance item in units ofmileage, time or the number of operations. The respective maintenanceinterval has an adjustable width “Delta” within which the maintenancehas to be carried out in the case of maintenance items with a controlfunction (tolerance range). One option “Connection” makes it possible todefine whether a maintenance item (for example brake disks), alwaysrequires another maintenance item to be carried out with it (for examplebrake linings). This reference may be defined on one side; that is, thereversal must then also be specified explicitly as it is not alwaysapplicable.

FIG. 8 shows an associated maintenance secondary item editor or workingstep editor. It can be used to edit and create maintenance secondaryitems. Each maintenance secondary item contains the parameters “Workingvalue” (i.e., costs) and “Working time”.

FIG. 9 shows, in an instantaneous recording, the current degree of wearof each selected maintenance item. If maintenance items are not yetsensed by load collectives or a sensor system, there is provision tosense speeded-up or slowed-down wear by means of a factor of a typifyingor classifying driver profile or driving style profile. Conversely, thelatter may be determined from the sensed or calculated wear.

FIG. 10 shows a screen representation of the current degree of wear ofall the selected maintenance items on a time axis using the individualdegrees of wear and maintenance intervals and tolerance ranges of themaintenance items.

FIG. 11 shows a screen representation in which the current degree ofwear of all the selected maintenance items is converted to the mileageor the distance traveled using the averaged number of miles traveled pertime.

FIG. 12 shows the optimum maintenance package for a predefined timedistance between two maintenance times, with the associated maintenanceitems, the working steps and the working steps which are saved throughcombination. By variably predefining the time distance (i.e., theminimum maintenance interval in units of time), it is possible tochoose, as it were in an infinitely variable fashion, between morecost-optimized variants (short time distances) or more time-optimizedvariants (long time distances), and the calculated optimum value ofa_(min) can also be set according to FIGS. 13 and 14. Here, cost gainsdue to combination and negative cost effects due to advancement ofmaintenance items in order to form maintenance packages as well as fixedstanding costs and standing costs per time are taken into account. Thecalculation amount, the costs of the optimum solution for a givenminimum maintenance interval per month and for the purposes ofcomparison, the costs when carrying out each maintenance operation whenthe respective maintenance item is due per month are displayed. Thelatter are hypothetical as, in practice, it is not always possible tolook for a service garage immediately when a maintenance item is due.

FIGS. 13 and 14 show screen representations of the results of theoptimization which is acquired by means of iteration, plotted over themonths, the timescales of the two representations differing. Theserepresentations are helpful as a decision aid for setting a suitableminimum maintenance interval. When the minimum maintenance interval islengthened, it may be that certain maintenance items no longer reach thesubsequent maintenance time. They are consequently advanced in time, andthe losses associated with them lead to a jump in the cost curve of theoptimum solution. Less than optimum maintenance intervals are thus to beconsidered as local minimums of the cost curve and preferably occurbefore the aforementioned sudden steps in the cost curve. The optimummaintenance interval is the absolute minimum of the cost curve.

Applied to a vehicle, the method according to the invention can beconfigured in a flexible manner, from small solutions with a smallnumber of maintenance items and simple models, such as linear wear andwear which is influenced by the driver profile, up to large solutionswith forty or more maintenance items and correspondingly large modelswith load collectives and integrated sensor systems. It is alsopossible, to always have implemented all the maintenance items, and onlyto enable the respective relevant ones by putting ticks in the screenrepresentations according to FIGS. 9 to 14 for the lists. For a specificvehicle, it is possible to transfer input maintenance items to othervehicles of the same type without difficulty. The editors of the graphicoperator control interfaces can also easily be used to create variantsfor similar types of vehicle. A simplified display interface which isvery clearly organized and thus convenient is made available to thesystem user in the vehicle and is used to display the determined optimummaintenance times and the extent of the respective maintenance operationfor its selection.

An approach is preferably selected in which overall standing costs aretaken into account over the maintenance period after summing once permaintenance period and also per failure time unit. Money is wasted byadvancing an item within the course of optimization and this is takeninto account in the optimization algorithm. The costs of a maintenancepackage are calculated from the costs of the individual maintenanceitems, it being taken into account that the combination of maintenanceitems by eliminating multiple working steps can result in cost savingswhich partially compensate for the moving forward of an item. It thenfollows from this that the sum of the cost saving through combination ofmaintenance items per time unit (negative value), on the one hand, andoverall standing costs per time unit plus wasted money through movingforward maintenance items per time unit, on the other, is the variablewhich determines the optimum with respect to the peripheral condition ofa predefined minimum interval between two maintenance deadlines. Ifiteration is performed over the predefined minimum interval between twomaintenance deadlines (for example in monthly intervals), a map of thecosts per time unit is obtained, plotted against the minimum intervalwhose minimum constitutes in each case the time and extent of themaintenance deadline, subsequent maintenance deadline, extent ofmaintenance and subsequent extent of maintenance which are optimum interms of costs. Here, the variable: sum of “costs per maintenanceitem/time” which is constant at the current time can also be added tothe cost function.

The above description of an advantageous exemplary embodiment makes itclear that the method according to the invention is advantageouslysuitable for monitoring individual system components which are subjectto wear, and defining (for a system to be maintained), the optimummaintenance times and the extent of respective maintenance operations,in advance using an optimization process. The system user can select amore cost-oriented or more time-oriented optimization using less thanoptimum solutions by manual correction. The maintenance costs can becalculated on an individual basis. By means of the framework algorithm,it is possible to include the system components individually in thisdefinition of the maintenance operations, and thus inter alia wearmodels, which are made available by component suppliers, can also betaken into account. Special maintenance operations, due for example tolegal requirements, can be taken into account.

Gains in terms of costs through combining maintenance items arecalculated in the optimization process together with the cost ofadvancing maintenance items in order to form maintenance packages, thecost of down time. The method according to the invention is, of course,suitable not only for the maintenance of vehicles but also for themaintenance of any system which requires regular maintenance with aplurality of maintenance items.

The foregoing disclosure has been set forth merely to illustrate theinvention and is not intended to be limiting. Since modifications of thedisclosed embodiments incorporating the spirit and substance of theinvention may occur to persons skilled in the art, the invention shouldbe construed to include everything within the scope of the appendedclaims and equivalents thereof.

1. A method for determining timing and scope of maintenance operationsfor a system having a plurality of maintenance items, said methodcomprising: predefining associated maintenance intervals and toleranceranges for implementing the maintenance items; predefining a minimummaintenance interval between an imminent maintenance operation and asubsequent maintenance operation; establishing a predicted time for anext subsequent maintenance operation, at the latest, at the time of atolerance range end point which occurs first after expiration of thepredefined minimum maintenance interval following the time of apreceding maintenance operation; and designating as maintenance items tobe carried out during the imminent maintenance operation at least thosemaintenance items whose tolerance range end points occur before thepredicted subsequent maintenance time.
 2. The method as claimed in claim1, wherein: each maintenance item is composed of at least onemaintenance secondary item; and when a maintenance sequence for arespective maintenance operation is created, maintenance secondary itemsfor the maintenance items to be carried out are tested with respect tothe possibility of combining them.
 3. The method as claimed in claim 1,wherein in designating maintenance items to be carried out during animminent maintenance operation, an optimization algorithm is implementedwhich includes advancing maintenance items from subsequent maintenanceoperations to earlier maintenance operations on a test basis, based on asecondary condition that the predefined minimum maintenance interval iscomplied with, and that an amount of the maintenance costs according toa cost function is optimized.